Modeling the Efficiency of Movement: From Individual Muscles to Whole Organism

Abstract

In the context of human movement, efficiency is defined as the ratio of mechanical energy output (work) to metabolic energy input [1,2]. It is straightforward to determine whole-body efficiency in a task such as pedaling a bicycle ergometer. In this case, work is computed from the ergometer load and pedaling cadence, and metabolic energy is determined from pulmonary gas exchange. It is also relatively straightforward to determine the efficiency of contraction in isolated muscle preparations, where the work done is easily measured, and energy input can be inferred from heat production or oxygen consumption. However, our understanding of the efficiency of muscle function during locomotion, and how this contributes to organismal efficiency, is incomplete [1]. The ability to determine efficiency of individual muscles as they perform work in vivo would greatly enhance our understating in this area. Experimental measurement of both work and metabolic energy consumption in muscles during dynamic activities is currently limited to isolated applications in non-human animals [3]. Similar data could be obtained using computational modeling and simulation techniques, provided that estimates could be obtained for both muscle work and muscle metabolic energy consumption. This non-invasive approach would open the door to investigations in humans as well as other species. Therefore, the primary purpose of this study was to determine efficiency at both the organismal and muscular levels for bicycle pedaling, using a musculoskeletal modeling approach. A secondary purpose was to identify factors that account for between-muscle differences in efficiency.